I have a question related to the normalization of data. I work for a company that maintains aircraft engines. These engines come in at different points during their lives for maintenance. In general, as the time on the aircraft engine increases, so does the cost of maintenance at a given point in time. Basically, what I am trying to do is normalize this data to remove the influence of the time on the aircraft's engine. Ultimately, what I would like to do is compare the maintenance cost of various fleets of aircraft engines (removing the influence of time) to determine if it is more costly to maintain engines that operate in different environments (sandy, dusty, pollution and smog, proximity to oceans (and salt)). Can anyone recommend any techniques that might help me remove the influence that time on the engine has on the maintenance cost.